1. Technical Field
The present invention relates in general to regulating power supplies, and in particular to a system and method for regulating the total power delivered from a parallel power supply. Still more particularly, the system and method of the present invention relate to dynamically adjusting the maximum current limit for each of a plurality of parallel power supplies in accordance with variations in available power supply capacity, such that the total available output power never exceeds a maximum allowable limit.
2. Description of the Related Art
Conventional power supply systems often utilize multiple, parallel power supply circuits for improved adaptability to load variations and also for increased reliability. Representative parallel power supply systems (referred to alternatively in the art as "N+1 power systems") are described in U.S. Pat. No. 5,319,536 (issued Jun. 7, 1994, to R. Malik), U.S. Pat. No. 5,266,838 (issued Nov. 30, 1993 to Gerner), and U.S. Pat. No. 5,745,670 (issued Apr. 28, 1998 to Linde).
FIG. 1 illustrates a conventional parallel power supply system 101 comprising three power supplies, PS1 102, PS2 103, and PS3 104 that together provide a source of power to electrical system 106. For safety reasons, a maximum allowable power output limit is imposed on each of PS1 102, PS2 103, and PS3 104, and also on the total power delivered from power supply system 101 to electrical system 106.
As seen in FIG. 1, each of the three power supplies in system 101 employs an individual current sense resistor (R.sub.CS 1 108, R.sub.CS 2 110, and R.sub.CS 3 112 for PS1 102, PS2 104, and PS3 106, respectively). R.sub.CS 1 108, R.sub.CS 2, and 110 R.sub.CS 3 112 are utilized to detect the level of current on each parallel supply line (I.sub.1 122, I.sub.2 124, and I.sub.3 126). An additional current sense resistor, R.sub.CS 114, is inserted within a power backplane 118 where the outputs of all three power supplies converge.
R.sub.CS 114 is utilized to sense the total supply current, I.sub.T 120, which is equal to the sum of I.sub.1 122, I.sub.2 124, and I.sub.3 126. Assuming that the Volt-Amp (VA) limit for electrical system 106 is the same as that for each power supply, the maximum allowable current level for I.sub.T 120 is three times the maximum level permitted for each individual power supply circuit (I.sub.1 122, I.sub.2 124, and I.sub.3 126). To ensure compliance with the maximum allowable VA limit, R.sub.CS 114 monitors the magnitude of I.sub.T 120, and if an excessive level of current (relative to V.sub.o 116) is detected, power supply system 101 is forced to shutdown. Current sense resistor R.sub.CS 114 must be a carefully selected resistor having a minimum voltage drop to avoid reducing rail-to-rail voltage. This low voltage drop requirement necessitates a precise, low resistance value for R.sub.CS 114. Such precision and low resistance characteristics dramatically increase the cost of R.sub.CS 114 and also require a low offset comparator. The utilization of an additional current sense resistor such as R.sub.CS 114 thus results in reduced power supply efficiency as well as increased cost of power supply system 101.
Modern circuits have much lower operating voltages than previous generations of integrated circuits. A current standard for state-of-the-art integrated circuits is 1.8 volts. Thus an additional voltage drop caused by the a conventional current sense resistor may result in an unacceptable decrease in the rail-to-rail voltage.
It would therefore be desirable to provide an indirect (non-resistive) method for determining total available parallel power supply capacity and dynamically adjusting the output current level of each parallel power supply in response thereto, such that the VA levels for all circuits within an N+1 power supply are maintained below the maximum allowable limit.